Stretch forming is a known process for shaping metal parts by stretching the metal beyond its elastic limit over a die. The sheet is deformed by tension forces into a shape matching that of the die, and the shape is retained due to being imparted when the sheet is stretched beyond its elastic limit. The general process is described in Volume 4 of "Metals Handbook" (8th Edition, 1969, pp. 239-45), and in U.S. Pat. Nos. 2,835,947, 3,073,373, 3,299,688, and 3,575,031, the disclosures of which are incorporated herein by reference.
An important factor in stretch forming is to control the tensile force applied to the sheet to insure that the elastic limit or yield point of the sheet is exceeded when the sheet is formed on the die. Insufficient tensile force prevents permanent deformation of the sheet into a desired shape, because the sheet elastically returns to its original shape when the force is removed. When the yield point is reached, additional force is normally aplied to insure formability of the part, but the resulting additional strain or elongation is carefully controlled to avoid sheet breakage.
A known technique for controlling stretch force is to detect the point at which the relationship between the variables of sheet stress and sheet strain becomes nonlinear. Departure from linearity indicates that the elastic limit or yield point has been exceeded, and that the sheet has been stressed to a point where permanent deformation can be imparted. Typical prior-art control systems are shown in U.S. Pat. Nos. 2,824,594, 2,945,527, and 2,999,528, the disclosures of which are incorporated herein by reference.
Sheet strain is typically measured by monitoring the positions of a pair of hydraulically actuated opposed jaws which grip the sheet and apply the stretching force. Alternatively, this measurement could be made on the sheet itself by conventional strain gages or similar transducers. This invention is not directed to sheet-strain measurement, but rather to an indirect method for measuring stress in the sheet by measuring elastic deformation of the jaw.
Stretch-press jaws are relatively massive structures with movable surfaces which are typically hydraulically driven to effect a tight clamping grip on opposite ends of the sheet to be formed. The entire jaw assemblies are in turn movable either linearly or in rotation with respect to each other to elongate and tension the sheet. In spite of the massive construction of the jaw assemblies, a small elastic deformation of each assembly arises from the reaction force of the tensioned sheet on the jaws.
In the practice of the invention, this elastic deformation of the jaw assembly is sensed optically by measuring the movement of a sharply focused light beam emitted from a source secured to the jaw assembly. Use of a photoelectric transducer enables light-beam movement to be converted to an electrical signal for comparison to a second and separately generated signal representing strain in the sheet.